1. Field of the Invention
This invention relates to an apparatus and method for biasing a high input impedance device or circuitry, such as a capacitive transducer for example. In particular, the invention relates to the operation and adjustment or calibration of the output voltage of a voltage supply apparatus during, for example, the use or testing of such high input impedance devices or circuits.
2. Description of the Related Art
Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as, for example, mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry, for example, are driving components to become smaller with higher functionality and reduced cost. For example, some mobile phones now require multiple microphones for noise cancelling, or accelerometers to allow inertial navigation, while maintaining or reducing the small form factor and aiming at a similar total cost to previous generation phones.
This has encouraged the emergence of miniature transducers. For example, in respect to speech applications, initially electret microphones were used to capture speech, but more recently micro-electrical-mechanical (MEMS) transducers have been introduced. MEMS transducers may be used in a variety of applications including, but not limited to, pressure sensing, ultrasonic scanning, acceleration monitoring and signal generation. Such MEMS transducers may be capacitive transducers comprising one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. Relative movement of these electrodes modulates the capacitance between them, which then has to be detected by associated electronic circuitry such as sensitive electronic amplifiers.
A complete MEMS device will typically comprise a MEMS transducer and associated electronic support circuitry including the amplifier.
As with the more conventional manufacturing technology that may be used for manufacturing the support circuitry, MEMS technology allows much of the manufacturing process to be performed on many devices at once, on a whole wafer containing thousands of devices, or even a batch of dozens of wafers. This fundamentally reduces production cost. Wafer-scale packaging techniques may also be used with similar benefits.
However, the production process contains many steps, not only the silicon-level processing steps for the transducer and the support circuitry, but also later steps, for example: placing the transducer on a common underlying substrate with the support circuitry; adding interconnects, such as bond wires for example, between the transducer and the electronics and from the electronics to terminals on the substrate; covering the assembly with protective material; and adding a case to cover the assembly. At each stage of manufacture, process variations may alter the characteristics, such as the acoustic sensitivity for example, of the capacitive (i.e. high impedance) transducer and/or the characteristics of the support circuitry, such as the power supply and amplifier circuitry. In particular, process variations can alter the overall sensitivity of a complete device such that a given acoustic stimulus will lead to different output signals being generated from nominally identical MEMS devices e.g. MEMS microphone devices.
Such variation in sensitivity is undesirable in the end application, particularly in applications where multiple high input impedance devices and circuits are used together (for example, the use of MEMS microphones in mobile phones or headphones that have noise cancelling circuitry that requires a plurality of said microphones). There is therefore a requirement for MEMS device manufacturers to produce devices with a small part-to-part spread of sensitivity, despite variability in the manufacturing processes. There is also a requirement for being able to calibrate, re-calibrate or normalise one or more MEMS devices, either individually or in relation to one another, either directly after production testing or sometime during use in the end application.
The sensitivity of a capacitive transducer may be proportional to the bias voltage imposed thereupon. So one method of adjusting the sensitivity is to adjust this bias voltage, generally in a final stage of manufacturing test, where a fixed physical stimulus is applied and the applied bias voltage adjusted until the required output amplitude is observed at the device output. The bias voltage is generally generated using a step-up voltage generator, such as a Dickson charge pump for example. However, the output voltage of open-loop voltage generators is generally variable with manufacturing tolerances, temperature and/or load current, so the bias voltage generated is not stable enough to generate a stable sensitivity. On the other hand, closed-loop voltage generators, where the voltage generator output is directly monitored and stabilised by a control loop, require control circuitry which increases the load on the output of the voltage generator itself, which adds components and hence cost, and also increases power consumption.